Under emergency situations, a driver may rapidly depress a brake pedal. However, the driver may not depress the brake pedal sufficiently or the driver may not maintain a sufficient force for a necessary period of time. Thus, to improve vehicle safety, some automatic brake assisting systems have been introduced in the market. Such known brake assisting systems employ a vacuum booster to automatically boost the braking force when the driver depresses the brake pedal rapidly.
Japanese Laid-Open Patent Publication No. H08-230634 published on Sep. 10, 1996, which corresponds to U.S. Pat. No. 5,727,852, discloses a brake assisting system. In the brake system described in this publication, a pump for anti-skid control or traction control is also used for boosting braking force in order to reduce the role of the vacuum booster. The system includes a wheel cylinder, a master cylinder, a main line connecting the master cylinder to the wheel cylinder, a modulator disposed in the main line to control pressure in the wheel cylinder, a reservoir connected to the wheel cylinder through the modulator, a first valve that opens and closes the main line between the master cylinder and the modulator, a pump connected between the reservoir and the wheel cylinder to supply pressurized brake fluid from the reservoir to the wheel cylinder, an auxiliary line connecting the inlet side of the pump to the master cylinder, and a second valve that opens and closes the auxiliary line.
Further, this known system also includes a pressure sensor to detect the output pressure from the master cylinder. The pump, the first valve and the second valve perform brake assistance under a proper control while the output pressure and/or the pressure increase ratio exceeds the predetermined value. To increase the pressure, the first valve is opened and the second valve is closed. Then, the pressure increase rate is controlled by turning the pressure pump on and off.
However, this system may suffer from disadvantages and drawbacks related to the performance of anti-skid control because the second valve is opened to increase the brake pressure.
During the brake assist control, the output pressure from the master cylinder is larger than the pressure stored in the reservoir because the driver has strongly depressed the brake pedal. Accordingly, the pump may discharge a lesser amount of brake fluid from the reservoir while the second valve is closed. Therefore, the reservoir may become full during a pressure decreasing period of the anti-skid control because the pump may not discharge all the brake fluid flowing into the reservoir.
In light of the foregoing, a need exists for a brake control apparatus that is not susceptible to the same disadvantages and drawbacks as those discussed above.
It would thus be desirable to provide a brake control apparatus that is able to simultaneously perform brake assist control and anti-skid control.
It would also be desirable to provide a brake control apparatus that is able to discharge brake fluid from a reservoir during anti-skid control.